1. Field of the Invention
The present invention relates to a drive transmission device for transmitting rotation to a rotating body with high accuracy, and an image forming apparatus and a process cartridge using the drive transmission device.
2. Description of the Related Art
In recent years, the image quality and image forming speed of image forming apparatuses, such as copying machines, printers, and facsimile machines, have improved markedly. For this reason, when rotational fluctuations occur in a photosensitive member and a rotating body included in an image forming unit such as a developing unit or a transfer unit, the image density of the resulting image tends to become uneven. Avoiding such density unevenness requires high rotation accuracies of the rotating bodies.
In particular, the rotational load of the developing unit is heavy, and therefore, it is effective to separate a drive transmission system of the developing unit from a drive transmission system of the photosensitive member, since rotational fluctuations of the photosensitive member greatly affect the image quality. On the other hand, in order to improve the image quality, it is important to accurately ensure a gap (developing gap) between the photosensitive member and a developing roller in the developing unit. Moreover, for extended working life and easy replacement, it is preferable that the image forming unit be removable from the apparatus body.
As a method and configuration that meet such demands for rotational accuracies of a plurality of rotating bodies and positional accuracy between the rotating bodies, it has been proposed to use a coupling to transmit the rotation from a driving system of the main body of an image forming apparatus to rotating bodies in an image forming unit.
An involute spline joint is known as a rotation transmission means for achieving high-accuracy rotation. FIG. 1 is a perspective view schematically showing the structure of a rotation transmission device using an involute spline joint. A photosensitive member 101 serving as an image bearing member is rotatably supported by a photosensitive member shaft 102. One end of the photosensitive member shaft 102 serves as a photosensitive-member-side joint 103 to which rotation is transmitted. In the rotation transmission device having this structure, the input from a photosensitive-member driving motor 106, such as a DC servo motor or a stepping motor, is transmitted to a driving-side joint 104 via a photosensitive-member driving shaft 105, and the photosensitive member 101 is rotated by engagement of the driving-side joint 104 with the photosensitive-member-side joint 103.
Further, coupling structures for independently transmitting the drive force to a photosensitive member and a developing unit or the like are disclosed. In one coupling structure, the drive force is transmitted to a photosensitive member by a joint shaped like a twisted triangular prism and to another image forming unit by a two-claw joint. In another coupling structure, the drive force is transmitted to the photosensitive member by an involute spline joint, and to another image forming unit by an Oldham coupling.
In the device shown in FIG. 1, high-accuracy rotation is achieved by using the involute spline joint to rotate the photosensitive member or another image forming unit. However, there is no attention paid to the positional accuracy between the rotating bodies. Further, in the above-described coupling structures, the rotation transmission systems are separately provided for the photosensitive member and another image forming unit, and the photosensitive member is accurately rotated using the involute spline joint or the joint shaped like a triangular prism. In contrast, the two-claw joint or the Oldham coupling is used for another image forming unit, and this may cause rotational fluctuations.